LOS ANGELES — A powerful storm out of the Gulf of Alaska will strike the Southland Thursday and Friday, generating fierce winds and threatening to trigger both flash flooding and mud flows down slopes stripped bare in wildfires, forecasters said today.

The rain will begin Thursday morning across San Luis Obispo and Santa Barbara counties, then spread to Ventura and Los Angeles counties in the afternoon and overnight, exiting L.A. County Friday morning, according to a National Weather Service statement.

“The warm nature of this storm system ahead of the cold front will bring periods of heavy rain,” it said. “Rainfall rates capable of triggering mud and debris flows within the recent burn areas will be possible. Rain will turn to showers by Friday morning.”

A flash flood watch will be in effect from Thursday afternoon through Friday morning in the so-called burn areas of L.A. County in the San Gabriel and Santa Monica mountains and the San Fernando, Santa Clarita and San Gabriel valleys, as well as in areas of Ventura County.

Also threatening the region are strong and potentially damaging winds. A high wind watch, projecting winds blowing or gusting at 58 miles per hour or more, will be in force from Thursday evening through Friday morning in the San Gabriel Mountains and the Antelope Valley. The wind in those areas is expected to blow at between 20 and 35 mph and gust at 60 mph before quickly tapering off Friday morning, forecasters said.

“Strong winds can make driving difficult, especially for drivers of high profile vehicles and vehicles towing trailers,” warned an NWS statement. “Winds this strong may down trees and power lines and cause property damage.”

The storm will be the biggest so far of the rainy season, which runs from October to May, said NWS meteorologist Andrew Rorke. Rainfall totals will range between a half-inch and 1.5 inches in coastal and valley areas and between 1 and 3 inches in the foothills and along south- and southwest-facing mountain slopes.

The NWS said snow levels are forecast to drop rapidly on Friday, falling to near 4,000 feet, but only minimal accumulation is expected over passes such as the Grapevine.

The storm will coincide with lower temperatures — the low 60s and high 50s. In contrast, today’s temperatures will be mostly in the mid- to high 60s.